Is It Stealth?

In his later life, Reimar Horten promoted the idea that the Horten Ho 229 V3 was intended to be built as a stealth aircraft, which would have placed this jet’s design several decades ahead of its time. Reimar Horten claimed that he wanted to add charcoal to the adhesive layers of the plywood skin of the production model to render it invisible to radar, because the charcoal “should diffuse radar beams, and make the aircraft invisible on radar” (Horten and Selinger 1983). This statement was published in his 1983 co-authored book Nurflugel (which translates as “only the wing”). While this statement refers to the never-made production model, it seems possible that the experimental charcoal addition could have been used on the Horten Ho 229 V3 prototype. The mere mention of early stealth technology sparked the imagination of aircraft enthusiasts across the world and spurred vibrant debate within the aviation community.

The stealth myth has been growing since the 1980s and was invigorated when the National Geographic Channel, in collaboration with Northrup Grumman, produced a documentary called "Hitler's Stealth Fighter" in 2009. The program featured the Horten Ho 229 V3 as a potential "Wonder Weapon" that arrived too late in the war to be used (Myth Merchant Films, 2009). The documentary also referred to the jet's storage location as "a secret government warehouse," which added to the mystique of this artifact. Since the airing of the documentary, public pressure has increased to remove the jet from its so-called secret government warehouse and put it on display. In fact, this secret warehouse is the Museum's Paul E. Garber Facility in Suitland, Maryland where a team of conservators, material scientists, a curator, and aircraft mechanic has been evaluating the aircraft.

Museum staff fabricated a steel stand to support and protect the center-section during the drive from the Paul E. Garber Facility in Suitland, MD, to the Steven F. Udvar-Hazy Center in Virginia. A protective cover was attached to the stand to completely enclose the artifact before shipment. Photo: Lauren Horelick

Searching for Charcoal

In 1983, Reimar Horten wrote in Nurflügel that he had planned to combine a mixture of sawdust, charcoal, and glue between the layers of wood that formed large areas of the exterior surface of the Ho 229 V3 production model. This was done to shield, he said, the "whole airplane" from radar as "the charcoal should absorb the electrical waves. Under this shield, then also the tubular steel [airframe] and the engines [would be] invisible [to radar]" (Horten and Selinger 1983, Russ Lee translation). While this statement refers to the never-made production model, it seems possible that the experimental charcoal addition could have been used on the Horten Ho 229 V3 prototype.

The Horten Ho 229 V3 from the front. Image: Ben Sullivan, 2013

In an effort to identify the presence of charcoal in the adhesive layer we first needed to determine which adhesive layer could contain the charcoal. In our adhesive research we discovered that there are two different adhesives used to assemble the plywood panels; urea formaldehyde and phenol formaldehyde. Only urea formaldehyde can accept additivies such as charcoal, and we found this material in thick layers that hold together the sub-assemblies of 5-ply boards (that are held together with phenol formaldehyde) as the image below illustrates. The second step to finding out if there is charcoal in the urea formaldehyde adhesive layer was to identify which analytical methods and protocols would best characterize this material.

Methods

A protocol was established for removing small samples of adhesives. Inclusions within the adhesives were carefully recorded, numbered, and photographed with high resolution images using a Hirox KH-7700 digital microscope.

In order to isolate the black particles, the adhesive matrix was crushed with a mortar and pestle and the resulting powder soaked in sodium hydroxide (pH 14) for several hours. Particles were pipetted onto a glass slide, allowed to dry, and the black particles separated out with tweezers. The extracted particles were characterized with X-ray diffraction (XRD) and Fourier-Transfer Infrared Spectroscopy (FTIR).

XRD was used to identify the black particles in the adhesive matrix. The instrument was a Rigaku D/Max Rapid Micro X-ray Diffractometer with copper target and operated at 50 kV, 40 mA, and 2.00 kv. Samples were mounted on a glass fiber using Elmers brand gel. This analysis was done at the Smithsonian's Museum Conservation Institute.

FTIR was carried out with a Thermo Nicolet 6700 FTIR spectrometer in attenuated total reflectance (ATR) mode with a single bounce, 45° Golden Gate ATR accessory with diamond crystal and an electronically cooled DTGS detector. Spectra were a co-addition of 64 scans at 4 cm-1 spectral resolution, and were ATR corrected. Each sample was analyzed three times. Analysis was carried out at the Museum Conservation Institute.

In addition, distribution of black particles in the adhesive matrix was observed and recorded. Thick pieces of the adhesive resin were prepared into thin sections (30-50 microns) by embedding the sample in clear polyester resin, cutting the cured resin sample with a diamond blade saw, and polishing with Microâ€“Mesh sheets. These were observed using optical microscopy.

Cross section of Horten plywood showing 5-ply assemblies of plywood held together with a thin amber colored adhesive layer, identified as phenol formaldehyde. The thick gray/black layer in between the 5-ply assemblies is urea formaldehyde. This material was used to lay up the pre-fabricated boards layer by layer to achieve the desired thickness of plywood on the aircraft. Urea formaldehyde is capable of accepting fillers and extenders such as charcoal. We focused our search for charcoal within this black colored adhesive layer. Image: Lauren Horelick, 2014

Samples of the adhesive were removed from in between the plywood layers and observed under the microscope. The adhesive appeared opaque black with transmitted light, though once the samples were crushed in a mortar and pestle and prepared into thin sections they appeared translucent and revealed a microcosm of colorful inclusions as seen in the images below.

This sample of urea formaldehyde adhesive came from inbetween the plywood on the Horten Ho 229 V3 and was photographed with transmitted light using a Hirox KH-7700 digital microscope. Note the colorful inclusions and the opaque white adhesive matrix. The image at the left shows the distribution of small black particles, and the image on the right shows measurements of the different inclusions. Image: Anna Weiss, 2014

This thin section of the adhesive collected from the Horten Ho 229 V3 shows the distribution of black particles within the adhesive matrix. Image: Anna Weiss

The image below shows a few of the extracted black particles. Each particle is about 200 microns in length (so, very small!).

Photomicrograph of tiny black particles extracted from the adhesive matrix. These samples were further characterized with XRD and FTIR. Image: Lauren Horelick, 2014

The black particles were extracted from the adhesive matrix and characterized using XRD and FTIR. The image below shows one of the extracted black particles prepared for characterization with XRD with the laser targeted on the sample. If the sample returns a spectrum that is crystalline then it will rule out charcoal. If it is amorphous (having no crystalline structure) then it could be many things including charcoal. XRD of our samples resulted in an amorphous spectrum.

Small black particle extracted from the urea formaldehyde adhesive layer of the Horten Ho 229 V3 and placed on a tiny spindle for analysis with XRD. The cross hairs in the image show where, exactly, the laser will focus on the small sample. If the sample produces a spectrum that is crystalline then it will rule out charcoal. If it's amorphous (having no crystalline structure) then it could be many things, including charcoal. This sample and others were amorphous. Analysis was carried out by Nicole Little at the Smithsonian's Museum Conservation Institute in 2013.

With FTIR the black particles resulted in a spectrum showing clear peaks. This is considered atypical for charcoal as the material more commonly blocks infrared light without producing spectral bands. In some cases, however, the method of charcoal processing can influence FTIR results, and in certain circumstances the material will produce a spectrum (Esteves et al., 2013, and Ilit Cohen-Ofri et al., 2006). In the spectrum of the extracted black particles we see a peak attributed to cellulose and hemicellulose at 3300, 2918, and 1029 cm-1. A peak around 1700 cm-1 is the C=O bond that may be from oxidation. Peaks around 1600, 1500 and 1250 cm-1 relate to a phenolic. Whether it is from lignin, a natural phenolic in the wood, or from the presence of a phenolic resin is uncertain (Ellen Nagy of Georgia Pacific Chemicals, personal communication 2014). The FTIR spectrum suggests that rather than discrete particles of charcoal within the adhesive matrix we are finding oxidized, or very aged wood.

FTIR spectra of black particles extracted from the adhesive matrix from two different locations on the Horten. Spectra by Dr. Odile Madden and Lauren Horelick in 2014 at Museum Conservation Institute.

The search for charcoal as a stealth material at this stage in the research appears inconclusive. However, a more macro view of the problem can be seen in the plywood cross-sections where there is a marked absence of a thick layer of anything except the adhesive. A cohesive and measurable layer of radar absorbing charcoal in between the plywood would suggest a concentrated effort at experimenting with stealth materials. We have a dilemma of scale in pursuit of identifying charcoal. The problem of looking for something tiny, like charcoal particles, within something huge, like an aircraft, is that it opens the door for speculative thoughts about looking and sampling in the right or wrong location. We characterized a material within the plywood adhesive that shares many commonalities with charcoal, but is not clearly and definitively charcoal. Reimar Hortenâ€™s published statement in the 1980s about wanting to add charcoal to the production model must be taken at face value as an idea. A production model was never fabricated, and the prototype does not show any clear evidence of charcoal as a stealth ingredient.

Stealth Expert Evaluation in 2008

In September 2008, a team of specialists from the Northrop Grumman Corporation analyzed the radar signature of the Horten 229 V3. Before Northrop bought Grumman in 1994, the company built 21 B-2 Spirit 'stealth' bombers for the U. S. Air Force. Northrop Grumman is recognized today for its expertise in technologies that make aircraft difficult to detect by radar or other means. Thomas Dobrenz, the director of Precision Engagement Technologies at Northrop, and Aldo Spadoni, the manager of Engineering Visualization also at Northrop, led a team that used sophisticated portable electronic equipment to measure how much radar energy the Horten jet reflected.

They presented their findings two years later at the 10th American Institute of Aeronautics and Astronautics Aviation Technology, Integration, and Operations (ATIO) Conference held September 13 through 15 in 2010, in Fort Worth, Texas. In a technical paper titled "Aviation Archeology of the Horten 229 v3 Aircraft" (AIAA 2010-9214), they summarized the goals and techniques used in their analysis:

"Modern RCS [radar cross-section, a measure of the detectability of an object with radar] tools were utilized to examine the one known Horten 229 V3 located at the Smithsonian facility in Washington, DC. A full-scale RCS model [of the Horten jet] was fabricated with modern techniques to simulate the aircraft as it would appear to electromagnetic energy. The original aircraft structure was constructed of a steel tube truss design covered by wood skins. The RCS model was constructed of wood to replicate the original design and complex parts were fabricated by modern techniques such as stereo-lithography. Northrop Grumman tested the full-scale replica at its outdoor RCS test facility in California at electromagnetic frequencies equivalent to the allied radar systems of World War II. Imaging techniques were utilized to understand the main RCS scattering sources of the Horten 229 V3."

Dobrenz and Spadoni explained that, "during our inspection of the Horten 229 at the Smithsonian museum, it appeared that a material similar to carbon black or charcoal was mixed in with the glue between the thin layers of the leading edge shape." After examining the aircraft, the men
analyzed sections of the leading edges of the jet's center section using a portable radar reflectometer called the Next Generation Sensor. They compared the Horten data to a plywood sample 2 centimeters (3/4 inches) thick, presumably a 'control sample,' against which the Horten data could be compared. The findings revealed that:

"The Ho 229 leading edge has the same characteristics as the plywood [control sample] except that the frequency [do not exactly match] and have a shorter bandwidth. This indicates that the dielectric constant of the Ho 229 leading edge is higher than the plywood test sample. The similarity of the two tests indicates that the design using the carbon black type material produced a poor absorber."

Dobrenz and Spadoni use the term 'absorber' to refer to the ability of the Ho 229 leading edge to absorb the radar signal rather than reflecting it back to the antenna receiver. More absorption means less reflected signal and greater stealth. The authors assumed in their paper that crafts persons used the "carbon black material" to lower the RCS, however, our technical study findings described above found no evidence of carbon black or charcoal in the Horten jet.